CSE461: Introduction to Computer Communication Networks

Justin Chan

TODO• Bits to signals and back again

• How does a wired protocol like Ethernet work?

• How do wireless protocols work?

• Machine learning + wireless

Ethernet

100Mb Gigabit 10Gb10Mb → → →

100Gb 200/400Gb Terabit40Gb → → →

20061980 1995 1999

2010 2010 2017 > 2020

2.5V

-2.5V

100 ns

0 0 0 1 1 0 0 1 0 0 0

Baseband Data

10MBit Ethernet uses Manchester encoding

100MBit (Fast Ethernet)

At least 2 bit transitions per block for clock

recovery

Scramble

Spread across multiple

frequencies to reduce

interference

Reduce bandwidth

16 hex symbols16 control symbols

MLT-34B/5B

1V

0V

-1V

4 bits

1→0→-1→0→1 0: no transition, 1: transition

4B/5B creates 125MBaud signal MLT-3 reduces fundamental frequency to 31.25MHz

Gigabit Ethernet

• 125MBaud per twisted pair.

• 500MBaud in total (2 bits per symbol)

• 4D-PAM5 (5 voltage levels, spread across 4 channels)

• Trellis modulation for parity bit

Spread spectrum Generate

parity bit

54=625 symbols in total

Encode 9 bit word: 8 data bits + 1 parity bit. 29=512 possible bitstrings

Remaining 113 symbols used for control or discarded

Generate symbols

Channels are interdependent

Modulate data onto a carrier wave Often 900MHz, 2.4GHz, 5GHz

Phase

Amplitude Frequency

Each symbol has a unique AMPLITUDE and PHASE

16QAM: 4 symbols per bit

Take the real value (x-axis)

and the imaginary value (y-axis)

Create In-phase and Quadrature (IQ)

waves

Multiply IQ waves by carrier wave frequency

and transmit

At receiver low-pass filter signal to get back I/Q

Noise Points blur

After fading and multipath

Attenuation (loss over distance) Points move closer

Many protocols use OFDM Multiple streams of data over a channel

More robust to interference

Frequency-selective fading

Square-Root raised cosine filter: matched filtering, less ISIViterbi decoder: undo convolutional code (error correcting code)

Channel estimation: Estimate channel taps from packet preamble

ISM bands• Reserved for industrial, scientific,

medical purposes

• Originally not for communication

• But used for short-range communication

• Longer range like cellular, satellite, amateur radio, FM radio use other reserved bands

• Common bands: 900MHz, 2.4GHz, 5GHz

Combine transmissions over several frequency channels.

Get Gigabit/s speeds

Packet Collisions

• CSMA

• Check channel, RTS/CTS

• Exponential backoff

• Standard for Wi-Fi, BLE, Zigbee

• What about other protocols?

Lora, Sigfox, NB-IoTLong range + can be decoded BELOW the noise floor

Lora range: > 10 miles

No MAC protocols yet!

Lora, Sigfox and Z-wave are proprietary (unlike Wi-Fi)

SNR=-10

SNR=-20

SNR=-30

Dimensionality reduction

Machine learning underneath the noise floor